The invention relates to a piezoelectrically actuated drive and adjustment element for transmitting pure translatory movements as well as rotational movements.
Piezoelectric drive and displacement means are known. In its simplest form, a piezoelectric element abuts against a non-displaceable support and pushes against a displaceable element. When an electric voltage is applied across the piezoelectric element it expands, displacing the displaceable element. Such setups are known in a number of varieties and transmit comparatively small variations in length of piezoelectric element by lever arrangements. The variations in length are not greater than 20-30 .mu.m at a maximum permissible excitation voltage even when individual elements are employed to form stacks with an overall height on an order of size of 20 mm. Such arrangements are used as precision drives, for example, in adjustment operations (refer to company brochure of the firm Physik Instruments, Waldbronn, 1994). Such arrangements, particularly when incorporated in devices, are disadvantageous because the piezoelectric element always has to operate against the lever structure and respective reset means and the piezoelectric element itself is subject to length variations due to varying influences of force and temperature. In particular, different thermal coefficients of expansion of the piezoelectric material and the material of a remainder of the device, such as steel, are disadvantageous. To compensate for such effects, the displacement element, when subject to temperature variations, is readjusted or is automatically regulated by means of a sensitive position measuring system which deteriorates displacement ranges available. Such measures also require considerable additional expenditures.
Furthermore, the above arrangements have a low internal rigidity due to the resetting means so that they only work quasi-statically at low response times.
Furthermore, units including piezoelectric elements are known which, by addition of single steps, obtain large displacement ranges (refer to: Muller, F. "Entwicklung eines piezoelektrischen StoBantriebs mit ausgepragter Klemmung", Univ. Diss., Dresden, 1989; Roscher, H. J. "Zur optimalen Gestaltung selbstandiger piezoelektrischen Stellantriebe mit magnetischer Klemmung fur Mikropositioniereinrichtungen". TH Diss. Karl-Marx-Stadt, 1980). In a so-called "inch-worm" device, at least three independently working piezoelectric elements are stacked to form one unit which, depending on the electrical excitation, differently contract or expand so that the unit performs movements or transmits movements to an element to be driven. Apart from a comparatively difficult setup, such units exhibit additional disadvantages. The individual step widths obtainable are comparatively small, about only 20 .mu.m. Furthermore, as concerns temperature variations, such devices do not have inherent compensation means. Although such devices ensure comparatively high clamping forces, a comparatively high operational wear results.
A further known basic modification of driving units which include piezoelectric elements provides the so-called impact drives which are embodied as flexure elements comprising two piezoelectric sheets to which opposite A.C. voltages are applied so that the points of action of these elements perform elliptic movements which act upon a drive means to initiate, for example, rotational movements of the latter. Particularly, when a plurality of such elements are employed, surprisingly high rotational speeds are obtained in some cases, however, only low rotational moments and forces, respectively, can be transmitted. Apart from low resonant frequencies, these elements also show a high wear.
The EP 0 574 945 A1 publication discloses a device for actuating a valve element which obviates the abovementioned problem of self-compensation for temperature variations by employing two piezoelectric stacks, each of which operates an actuation lever of a valve. This arrangement produces tilting movements and/or lifts on both arms connected to the stacks via respective fulcrums. In operation, the piezoelectric stack elements substantially perform free movements wherein rotational and tilting movements superimpose. This construction exhibits poor rigidity with respect to external forces and requires additional supports.